The invention relates to a mass flowmeter, in particular for detecting an air mass flow in an intake duct of an internal combustion engine.
European Patent Application 0 458 998 A1, corresponding to U.S. Pat. No. 5,253,517, discloses a mass flowmeter having a housing in which a flow channel is constructed, and in which a flow straightener or rectifier is inserted upstream of a sensor element. The flow straightener or rectifier includes a honeycomb member and a ring which projects over the honeycomb in the flow direction and in which a grille that generates micro-eddies is embedded at a spacing from the honeycombs. The grille smooths the flow along the entire cross section of the flow straightener or rectifier. That ensures that the flow in the region of the sensor element is well conditioned, and a sensor signal which is generated by the sensor element precisely and accurately represents the air mass flow in the flow channel. However, the small-mesh grille has the disadvantage of causing a pressure loss, and thus leading to worsening of the efficiency of an internal combustion engine in the intake duct of which the mass flowmeter can be disposed. Furthermore, the grille can also be easily contaminated by particles present in the air flow, that can stick in the grille and thus lead to disturbances in the flow which then in turn entail a falsification of the sensor signal.
The grille further has the disadvantage of experiencing fatigue in the case of a lengthy operating period of the mass flowmeter accompanied by high vibrational loads such as frequently occur in the field of motor vehicle engineering, and can fail mechanically. Moreover, the insertion of the grille into the ring of the honeycomb body is complicated, and therefore expensive.
It is accordingly an object of the invention to provide a mass flowmeter, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type in such a way that firstly, it is easy to produce and secondly, it generates a precise sensor signal over an entire measuring range, with pressure loss in a flow channel being minimized.
With the foregoing and other objects in view there is provided, in accordance with the invention, a mass flowmeter, comprising a sensor element and a flow straightener or rectifier including a honeycomb member having at least one honeycomb aligned with the sensor element in a flow direction. The honeycomb member includes two honeycomb walls enclosing an angle having a bisector axis. The sensor element is positioned relative to the bisector axis to cause a mass flow to correspond approximately to a mean value of a wave-shaped air flow in direction of the bisector axis when the flow is about to change over from laminar to turbulent flow.
In accordance with another feature of the invention, the angle enclosed by the two honeycomb walls is smaller than 90 degrees and, in particular, is between 20 and 60 degrees.
In accordance with a further feature of the invention, the angle is a function of a positional tolerance of the sensor element with reference to the bisector axis, and of a prescribed maximum discontinuity in a sensor signal of the sensor element upon transition of the flow from laminar to turbulent.
In accordance with a concomitant feature of the invention, the at least one honeycomb is rhomboidal and includes two further honeycomb walls enclosing an obtuse angle.
The invention proceeds from the surprising finding that only shortly before the changeover from laminar to turbulent flow, which is a function of the mass flow and further parameters, the flow along a chord of the circular cross section of the flow channel has a wave-shaped course about the mean value of the sensor signal of the air mass flow. The amplitude of the wave-shaped oscillation can be over 50% of the mean value in this case. The sensor element is positioned with reference to the bisector axis of an angle enclosed by two honeycomb walls in such a way that the air mass flow there corresponds approximately to the mean value of the wave-shaped air flow in the direction of the bisector axis when the flow is about to change over from laminar to turbulent flow. Thus, it is ensured in a simple way that the sensor signal does not cause any discontinuity upon changeover of the flow from laminar to turbulent. Such a discontinuity would entail a large measuring inaccuracy.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a mass flowmeter, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.